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PubMed2004

Serine:glyoxylate aminotransferases from maize and wheat leaves: purification and properties.

Photorespiratory enzyme serine:glyoxylate aminotransferase (SGAT, EC 2.6.1.45) was purified from green parts of seedlings of two Gramminae species with different photosynthetic pathways, maize (Zea mays L., C(4) species) and wheat (Triticum aestivum L., C(3) species). The preparation from wheat was homogeneous as judged by SDS-PAGE with silver staining for proteins; however, the same method revealed approximately 9% contamination in a highly purified maize preparation. Molecular masses of SGAT from maize and wheat were estimated by SDS-PAGE to be 44.1 and 44.6 kDa, respectively. C(4) enzyme exhibited a specific activity in homogenates that was seven times lower than wheat, and this was associated with lower K (m) values for all substrates examined as well as a more than two times lower turnover number k (cat) with serine and glyoxylate as a pair of substrates. In contrast, the ratio of the turnover number to K (m)(Ser)(k (cat)/K (m)(Ser)) for C(4) aminotransferase proved to be about two times higher than for C(3) aminotransferase. The sensitivity of two enzymes to some inhibitors, especially aminooxyacetate, was different and they also differed with respect to thermal stability and pH optimum - the maize enzyme required 0.6 unit higher pH (8.6) for maximal activity and was more heat-resistant.

Journal Article↗

PubMed1968

Factors affecting the premature induction of tyrosine aminotransferase in foetal rat liver.

1. Premature delivery of foetal rats by uterine section results in the rapid appearance of tyrosine aminotransferase activity in foetal liver, after an initial lag period of 3-6hr. 2. The premature induction of activity is completely repressible by actinomycin D given soon after delivery and partially repressible by puromycin and amino acid analogues. 3. Glucagon injections into foetal rats in utero lead to production of tyrosine aminotransferase in the foetal liver, but adrenalin and nor-adrenalin are without effect. 4. Injections of glucose, galactose, fructose and mannose into prematurely delivered rats repress the development of tyrosine aminotransferase activity about 50% when they are given 2hr. after delivery, but glucose has no significant effect when injected at delivery. 5. The results are discussed in relation to current hypotheses on the role of hormones in enzyme induction in foetal development.

Animals↗

PubMed1972

The effects of salicylate on the activity of rat liver tyrosine-2-oxoglutarate aminotransferase in vitro and in vivo.

1. Salicylate, in concentrations of 0.25mm and above, enhances the basal activity of tyrosine-2-oxoglutarate aminotransferase in homogenates of rat liver incubated in the absence of added pyridoxal 5'-phosphate (endogenous activity). The effect is decreased by increasing the concentration of the cofactor. 2. The intraperitoneal administration of sodium salicylate enhances the activity of rat liver tyrosine aminotransferase; the major effect during the first hour being on the enzyme in the absence of added pyridoxal phosphate. Actinomycin D prevents the induction of the enzyme by cortisol and tryptophan. Induction by pyridoxine or salicylate is 50% inhibited by actinomycin D. The effects of the injections of various combinations of cortisol, pyridoxine and salicylate were also studied in the absence or presence of actinomycin D. 3. It is suggested that salicylate induces rat liver tyrosine aminotransferase by displacing its protein-bound cofactor and that a cofactor-type induction of the hepatic enzyme occurs in pyridoxine-treated rats.

Adrenalectomy↗

PubMed1974

Regulation of hepatic L-serine dehydratase and L-serine-pyruvate aminotransferase in the developing neonatal rat.

1. The activities of l-serine dehydratase and l-serine-pyruvate aminotransferase were determined in rat liver during foetal and neonatal development. 2. l-Serine-pyruvate aminotransferase activity begins to develop in late-foetal liver, increases rapidly at birth to a peak during suckling and then decreases at weaning to the adult value. 3. l-Serine dehydratase activity is very low prenatally, but increases rapidly after birth to a transient peak. After a second transient peak around the time weaning begins, activity gradually rises to the adult value. Both of these peaks have similar isoenzyme compositions. 4. In foetal liver both l-serine dehydratase and l-serine-pyruvate aminotransferase activities are increased after injection in utero of glucagon or dibutyryl cyclic AMP. Cycloheximide or actinomycin D inhibited the prenatal induction of both enzymes and actinomycin D blocked the natural increase of l-serine dehydratase immediately after birth. Glucose or insulin administration also blocked the perinatal increase of l-serine dehydratase. 5. After the first perinatal peak of l-serine dehydratase, activity is increased by cortisol and this is inhibited by actinomycin D. After the second postnatal peak, activity is increased by amino acids or cortisol and this is insensitive to actinomycin D inhibition. Glucose administration blocks the cortisol-stimulated increase in l-serine dehydratase and also partially lowers the second postnatal peak of activity. 6. The developmental patterns of the enzymes are discussed in relation to the pathways of gluconeogenesis from l-serine. The regulation of enzyme activity by hormonal and dietary factors is discussed with reference to the changes in stimuli that occur during neonatal development and to their possible mechanisms of action.

Animals↗

PubMed1977

Factors influencing the activity of ornithine aminotransferase in isolated rat liver mitochondria.

1. The characteristics of ornithine catabolism by the aminotransferase pathway in isolated mitochondria were determined. 2. Ornithine synthesis from glutamate and glutamate gamma-semialdehyde produced by the oxidation of proline was studied. No ornithine was formed in the absence of rotenone. 3. The mechanism of ornithine transport was reinvestigated, and the existence of an ornithine+/H+ exchange system postulated. 4. The kinetics of ornithine transport, ornithine catabolism in intact mitochondria and ornithine aminotransferase activity in solubilized mitochondria were compared. It is concluded that ornithine aminotransferase activity in liver mitochondria is rate-limited by the transport of ornithine across the mitochondrial membrane, and that this enzyme is involved primarily in ornithine degradation rather than ornithine synthesis.

Animals↗

PubMed1978

Subcellular distribution of pyruvate (glyoxylate) aminotransferases in rat liver.

The distribution of pyruvate (glyoxylate) aminotransferases in the particulate fraction of rat liver homogenates was examined by centrifugation in a sucrose density graident. Aminotransferase activities towards serine, phenylalanine and histidine with pyruvate and those towards phenylalanine and histidine with glyoxylate were nearly identically distributed. Some 50-55% of the particulate activity was localized in the peroxisomes and the remainder in the mitochondria. Most of alanine-glyoxylate aminotransferase activity was localized in the mitochondria, with some activity in the peroxisomes. Glucagon injection resulted in increases of these enzyme activities in the mitochondria, but not in the peroxisomes.

Animals↗

PubMed1980

The effect of cytosine arabinoside and bromodeoxyuridine on the appearance of tyrosine aminotransferase in cultured foetal hepatocytes of the rat.

1. The acquisition of dexamethasone-inducibility of tyrosine aminotransferase activity by hepatocytes cultured from 15-day-foetal rat liver is blocked in the presence of cytosine arabinoside. 2. Similar results are obtained in the presence of bormodeoxyuridine. 3. No effects on steroid-inducibility of tyrosine aminotransferase are obtained with either of the above compounds in hepatocytes cultured from 19-day-foetal liver. 4. the inhibitory effects of the agents are substantially reversed after their removal from the culture medium. 5. The effects of bromodeoxyuridine suggest that cell differentiation, with respect to tyrosine aminotransferase-inducibility, occurs in cultures of 15-day-doetal hepatocytes. 6. The effects of cytosine arabinoside suggest that such an event is dependent on mitosis.

Animals↗

PubMed1982

Development of glutamate:glyoxylate aminotransferase in the cotyledons of cucumber (Cucumis sativus) seedlings.

Glutamate:glyoxylate aminotransferase had been reported to be present exclusively in the peroxisomes of plant leaves and to participate in the glycollate pathway in leaf photorespiration (Tolbert (1971) Annu. Rev. Plant Physiol. 22, 45-74]. Glutamate:glyoxylate aminotransferase activity was already present in the etiolated cotyledons of cucumber (Cucumis sativus) seedlings, and increased during greening. The enzyme was present only in the cytosol of the etiolated cotyledons and appeared in the peroxisomes during greening. The enzyme was purified to homogeneity from the cytosol of the etiolated cotyledons and from the peroxisomes of the green cotyledons of cucumber seedlings. The two enzyme preparations had nearly identical enzymic and physical properties. On the basis of these findings, roles of glutamate:glyoxylate aminotransferase in the glycollate pathway in photorespiration, and the mechanism of its appearance in the peroxisomes during greening, are discussed.

Glutamates↗

PubMed1982

Effects of protein-degradation inhibitors on the inactivation of tyrosine aminotransferase, tryptophan oxygenase and benzopyrene hydroxylase in isolated rat hepatocytes.

The following three potent inhibitors of hepatocytic proteolysis were investigated to see if they would inhibit the intracellular inactivation of enzymes: chymostatin and leupeptin (proteinase inhibitors) and methylamine (a lysosomotropic weak base). Chymostatin inhibited the inactivation of two of the three enzymes tested: tyrosine aminotransferase (EC 2.6.1.5) and tryptophan oxygenase (tryptophan 2,3-dioxygenase, EC 1.13.11.11). Leupeptin had no effect on any of the enzymes, whereas methylamine had only a weak inhibitory effect on tyrosine aminotransferase inactivation. Apparently proteolytic cleavage (probably by a non-lysosomal proteinase, since only chymostatin is effective) is involved in the inactivation of tyrosine aminotransferase and tryptophan oxygenase. The third enzyme, benzopyrene hydroxylase (flavoprotein-linked mono-oxygenase, EC 1.14.14.1), is probably inactivated by a non-proteolytic mechanism.

Animals↗

PubMed1982

The subcellular distribution of rat liver serine-pyruvate aminotransferase.

1. The subcellular distribution of L-serine-pyruvate aminotransferase activity in rat liver was investigated. About 80% was recovered from cell-free homogenates in a 'total-particles' fraction and the remainder in the cytosol. 2. Subfractionation of the particles by differential sedimentation and on sucrose density gradients showed a distribution for serine-pyruvate aminotransferase activity closely matching that observed for mitochondrial marker enzymes. 3. A study of the solubilization of enzymes from combined subcellular particles by digitonin at various concentrations also indicated a common subcellular location for serine-pyruvate aminotransferase and established mitochondrial enzymes. 4. The increase in liver serine-pyruvate amino-transferase activity induced by glucagon injection was accounted for as an increased mitochondrial activity.

Animals↗

PubMed1996

Glutamate-1-semialdehyde aminotransferase from Sulfolobus solfataricus.

Glutamate-1-semialdehyde aminotransferase (GSA-AT) from the extremely thermophilic bacterium Sulfolobus solfataricus has been purified to homogeneity and characterized. GSA-AT is the last enzyme in the C5 pathway for the conversion of glutamate into the tetrapyrrole precursor delta-aminolaevulinate (ALA) in plants, algae and several bacteria. The active form of GSA-AT from S. solfataricus seems to be a homodimer with a molecular mass of 87 kDa. The absorption spectrum of the purified aminotransferase is indicative of the presence of pyridoxamine 5'-phosphate (PMP) cofactor, and the catalytic activity of the enzyme is further stimulated by addition of PMP. 3-Amino-2,3-dihydrobenzoic acid is an inhibitor of the aminotransferase activity. The N-terminal amino acid sequence of GSA-AT from S. solfataricus was found to share significant similarity with the eukaryotic and eubacterial enzymes. Evidence is provided that ALA synthesis in S. solfataricus follows the C5 pathway characteristic of plants, algae, cyanobacteria and many other bacteria.

Amino Acid Sequence↗

PubMed1996

Regulation of tyrosine aminotransferase gene expression by glucocorticoids in quiescent and regenerating liver.

Following 70% hepatectomy, the induction of tyrosine amino-transferase mRNA by glucocorticoids was marginal at 1.5 h, significantly impaired between 3 and 8 h and, at 16 h post-hepatectomy, reached a value approx. 5-fold the basal level, similar to the level observed in quiescent liver. The fold induction of the mRNA was accounted for by a similar fold activation of transcription of the gene by glucocorticoids in regenerating but not in quiescent liver; in the latter, activation of transcription was marginal in spite of glucocorticoid-induced hypersensitivity to cleavage by DNase I at the glucocorticoid-dependent enhancer of the gene. The possibility that in quiescent liver glucocorticoids act at a transcriptional step beyond initiation, increasing the rate of elongation or overcoming a blockage in elongation, was excluded. However, a similar fold induction was determined for total and nuclear tyrosine aminotransferase mRNA in the presence of glucocorticoids, suggesting that in quiescent liver glucocorticoids promote efficient maturation of the tyrosine aminotransferase primary transcript. Thus a glucocorticoid-induced nuclear post-transcriptional up-regulation apparently compensates for impaired activation of transcription of the tyrosine aminotransferase gene by glucocorticoids in quiescent liver.

Animals↗

PubMed1988

Further studies on the activity and subcellular distribution of alanine:glyoxylate aminotransferase in the livers of patients with primary hyperoxaluria type 1.

1. The activity of alanine:glyoxylate aminotransferase (AGT; EC 2.6.1.44) has been measured in the unfractionated livers of 20 patients with primary hyperoxaluria type 1 (PH1), three patients with other forms of primary hyperoxaluria and one PH1 heterozygote. The subcellular distribution of AGT activity was examined in four of the PH1 livers and in the liver of the PH1 heterozygote. 2. The mean AGT activity in the unfractionated PH1 livers was 12.6% of the mean control value. The activities of other aminotransferases and the peroxisomal marker enzymes were normal. When corrected for cross-over from glutamate:glyoxylate aminotransferase (GGT; EC 2.6.1.4), the mean AGT activity in the PH1 livers was reduced to 3.3% of the control values. 3. The livers from a patient with primary hyperoxaluria type 2 (D-glycerate dehydrogenase deficiency) and one with an undefined form of primary hyperoxaluria (possibly oxalate hyperabsorption) had normal AGT levels. The livers of a very mild PH1-type variant and a PH1 heterozygote had intermediate levels of AGT activity. 4. Subcellular fractionation of four PH1 livers by sucrose gradient isopycnic centrifugation demonstrated a complete absence of peroxisomal AGT activity. The subcellular distribution of the residual AGT activity was very similar to that of GGT activity (i.e. mainly cytosolic with a small amount mitochondrial). There were no alterations in the subcellular distributions of any of the peroxisomal marker enzymes. The subcellular distribution of AGT activity in the PH1 heterozygote liver was similar to that of the control (i.e. mainly peroxisomal).

Adolescent↗

PubMed2001

Peroxisomal alanine : glyoxylate aminotransferase (AGT1) is a photorespiratory enzyme with multiple substrates in Arabidopsis thaliana.

At least two glyoxylate aminotransferases are hypothesized to participate in the steps of photorespiration located in peroxisomes. Until recently, however, genes encoding these enzymes had not been identified. We describe the isolation and characterization of an alanine : glyoxylate aminotransferase (AGT1, formerly AGT) cDNA from Arabidopsis thaliana. Southern blot analysis confirmed that Arabidopsis AGT1 is encoded by a single gene. Homologs of this class IV aminotransferase are also known in other plants, animals, and methylotrophic bacteria, suggesting an ancient evolutionary origin of this enzyme. AGT1 transcripts were present in all tissues of Arabidopsis, but were most abundant in green, leafy tissues. Purified, recombinant Arabidopsis AGT1 expressed in Escherichia coli catalyzed three transamination reactions using the following amino donor : acceptor combinations: alanine : glyoxylate, serine : glyoxylate, and serine : pyruvate. AGT1 had the highest specific activity with the serine : glyoxylate transamination, and apparent Km measurements indicate that this is the preferred in vivo reaction. In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. Sequence analysis of the photorespiratory sat mutants revealed a single nucleotide substitution in the AGT1 gene from these plants. This transition mutation is predicted to result in a proline-to-leucine substitution at residue 251 of AGT1. When this mutation was engineered into the recombinant AGT1 protein, enzymatic activity using all three donor : acceptor pairs was abolished. We conclude that Arabidopsis AGT1 is a peroxisomal photorespiratory enzyme that catalyzes transamination reactions with multiple substrates.

Amino Acid Sequence↗

PubMed2006

Low prevalence of chronic hepatitis C, but high prevalence of elevated aminotransferases in a cohort of 2026 patients referred for orthopaedic surgery in the eastern part of Germany.

INTRODUCTION: The prevalence of chronic hepatitis C in Germany is about 0.2 - 0.4 %. However, there seems to be regional differences between western and eastern states of the country. Thus, the present study analysed the prevalence of chronic hepatitis C in a cohort of orthopaedic patients in Thuringia. METHODS: Tests for antibodies against hepatitis C virus (anti-HCV) were performed on serum samples of 2026 patients (1183 females, 843 males) admitted for orthopaedic surgery to a university hospital in Thuringia. If anti-HCV was positive, serum was tested for HCV-RNA by polymerase chain reaction (PCR). For the sake of anonymity only age and gender were reported in all patients. In 1465 cases, values of alanine (ALT) and aspartate (AST) aminotransferases were additionally available. The low HCV prevalence was confirmed in a second cohort of orthopaedic patients (n = 929, 599 females, 330 males) investigated at a university hospital in Saxonia. RESULTS: In the Thuringian cohort, anti-HCV was detectable in 12/2026 (0.6 %) individuals (10 females (0.85 %) and 2 males (0.24 %: p = 0.14 %). HCV-RNA was positive in 3/10 of anti-HCV positive females (0.15 % of the study cohort). HCV infection was already known in two cases. Anti-HCV positive patients seemed to be older than anti-HCV negative individuals (64.25 vs. 59.48 years; p = 0.17), as well as HCV-RNA positive cases compared to non-viraemic patients (66.3 vs. 63.6 years; p = 0.32). All HCV-RNA positive females had elevated ALT values. However, ALT and AST were also elevated in 18.2 % and 11.7 % of anti-HCV negative individuals. There was no significant difference between males and females (p = 0.32). In the Saxonian cohort none of 929 individuals were HCV positive. CONCLUSION: The prevalence of chronic hepatitis C is low in the investigated cohorts of orthopaedic patients in Thuringia and Saxonia. However, elevation of aminotransferases occurs surprisingly often. The reasons for elevated aminotransferases and a reliable analysis of the HCV prevalence in different subgroups of the Eastern German population require further evaluation.

Cohort Studies↗

PubMed1971

Expression of differentiated functions in hepatoma cell hybrids: reappearance of tyrosine aminotransferase inducibility after the loss of chromosomes.

HYBRIDS FROM A CROSS OF RAT HEPATOMA CELLS WITH DIPLOID EPITHELIAL CELLS FROM RAT LIVER HAVE BEEN STUDIED WITH RESPECT TO KARYOTYPE AND EXPRESSION OF TWO FUNCTIONS LIMITED TO THE HEPATOMA PARENT: high level of the enzyme tyrosine aminotransferase (EC 2.6.1.5; L-tyrosine:2-oxoglutarate aminotransferase) and its inducibility with steroid hormones. The hybrids that contain the complete chromosomal complements from both parents show low enzyme activity and no inducibility. One hybrid clone, and all of its derivatives, which have lost 30-40% of the chromosomes initially present, show enzyme inducibility. Induction of tyrosine aminotransferase in the hepatoma and hybrid cells responds similarly to inhibition by cycloheximide and actinomycin D, and to steroid concentration. The enzymes from induced and noninduced hepatoma cells and from induced hybrid cells are similar in heat sensitivity and intracellular distribution; those from noninduced hybrid and diploid rat epithelial cells are different.

Animals↗

PubMed1972

Synthesis of inducible tyrosine aminotransferase in a cell-free extract from cultured hepatoma cells.

A cell-free extract from cultured rat hepatoma cells is described that incorporates amino acids into chains of tyrosine aminotransferase that are identical, by several criteria, with tyrosine aminotransferase from rat liver. Extracts from steroid-induced cells are 6-to 10-times more active for tyrosine aminotransferase synthesis than extracts from uninduced cells, although the total incorporation rates are identical.

Amino Acids↗

PubMed1972

Evidence for reversible inactivation of induced tyrosine aminotransferase in rat liver in vivo.

Induction of tyrosine aminotransferase (EC 2.6.1.5) with cortisol is followed by a rapid phase of inactivation of the enzyme, during which administration of puromycin causes a sharp increase in enzyme activity. The increase is not due to puromycin-resistant synthesis of protein, since precipitation of the enzyme with antibodies to tyrosine aminotransferase after pulse labeling inhibited incorporation of radioactive amino acids. Reactivation of the enzyme was specific, since the structurally similar dimethylaminopurine and purine aminonucleoside were ineffective. An action of puromycin other than its capacity to inhibit protein synthesis was required for reactivation, since cycloheximide did not demonstrate such an effect. In vitro studies with free and bound polysomes, isolated during the phase of inactivation, indicated that reactivation by puromycin was not due to release of nascent tyrosine aminotransferase peptides. We conclude that the enzyme is initially reversibly inactivated; therefore, the overall degradation of this intracellular protein is a multistep phenomenon.

Adrenalectomy↗